The present invention relates to an improved process for the continuous preparation of 2,6-dinitro-N-alkyl-anilines of the general formula (1): ##STR1## in which R.sub.1 and R.sub.2 represent independently a saturated or unsaturated hydrocarbon radical containing from 1 to 6 atoms of carbon, or a cycloalkyl radical containing from 3 to 6 atoms of carbon in the ring, and in which R.sub.3 represents an atom of hydrogen, an alkyl radical containing from 1 to 4 atoms of carbon, or an alkyl radical with from 1 to 4 atoms of carbon in which the hydrogen atoms are partially or completely substituted by halogen atoms.
Among the 2,6-dinitro-N-alkyl-anilines represented by the general formula (1) are numerous herbicides and, among these, the best known are: 2,6-dinitro-4-isopropyl-N,N-dipropyl-aniline (Isopropalin); 2,6-dinitro-4-trifluoromethyl-N,N-dipropylaniline (Trifluralin); 2,6-dinitro-4-trifluoromethyl-N-cyclopropylmethyl-N-propyl-aniline (Profluralin); and 2,6-dinitro-4-trifluoromethyl-N-butyl-N-ethyl-aniline (Benfluralin). The herbicidal characteristics of these compounds and of other similar compounds are described in U.S. Pat. Nos. 3,257,190 and 3,403,180.
In particular the compounds corresponding to the general formula (1) in which R.sub.3 is a trifluoromethyl group are generally prepared from 2,6-dinitro-4-trifluoromethyl-chlorobenzene by substitution of the chlorine atom by reaction with an amine NHR.sub.1 R.sub.2, in which R.sub.1 and R.sub.2 have the meaning already indicated above.
In practice, the reaction is carried out according to the general scheme: ##STR2## by reacting 2,6-dinitro-4-trifluoromethyl-chlorobenzene with two equivalents of the amine (which thus also acts as the acceptor for the hydrochloric acid which is formed as a by-product of the reaction) in a discontinuous operation, to obtain the desired 2,6-dinitro-4-trifluoromethyl-N-alkyl-aniline.
In the literature, and particularly in patent literature, other processes are described for the preparation of 2,6-dinitro-N-alkyl-anilines represented by the general formula (1).
Thus, for example, according to U.S. Pat. No. 3,403,180, 2,6-dinitro-chlorobenzene, possibly substituted in the 4-position of the benzene ring, and two equivalents of the chosen amine are reacted by heating the reaction mixture for a time of 2 to 4 hours. This method is disadvantageous in that the use of the amine as the acceptor for hydrochloric acid makes necessary the separation of the hydrochloride of the amine from the reaction mixture and the subsequent treatment of the said hydrochloride with an alkali metal hydroxide (generally sodium hydroxide) to recover the said amine. In French Pat. No. 2,051,301, an attempt was made to avoid the disadvantages resulting from the presence of the amine as the acceptor for hydrochloric acid by using a mineral base, such as sodium hydroxide or sodium carbonate, for this purpose. According to the process described in this French Patent, the 2,6-dinitro-chlorobenzene is fed into a dilute solution of sodium hydroxide (about 8% concentration) and the amine is gradually added to the suspension obtained, the reaction temperature being maintained at values less than about 15.degree. to 20.degree. C. by cooling the reaction mass. After this addition, which requires a time of the order of three hours, the mass is heated to 80.degree. C. and left to react for a further two hours. This method also has numerous disadvantages. First of all, the total reaction time is of the order of 5 hours and, furthermore, there are the complexities resulting from the cooling and the subsequent heating of the reaction mass. Moreover, since the reaction is carried out in a two-phase system, very strong agitation is necessary in order to achieve satisfactory contact between the aqueous alkaline phase and the organic phase. Finally, the low concentration of sodium hydroxide in its solution, dictated by the need to reduce the side reactions, necessitates the use of large reaction volumes and hence results in a low productive capacity, this being understood as the quantity of desired product per unit of useful volume of the reactor. It should also be noted that the conversion into the desired product is of the order of 91-93% according to the patent under discussion.
This shows that, under the reaction conditions used, an amount of 2,6-dinitro-chlorobenzene of the order of 7 to 9% is lost owing to side reactions, which, on the one hand, renders the process hardly economic, and, on the other hand, aggravates the problems resulting from the disposal of the aqueous effluent.
Hence the known art does not allow 2,6-dinitro-N-alkyl-anilines to be produced with high values of the yield and selectivity.
Moreover, the known art does not teach any method of achieving high values of the yield and selectivity in a reaction system which is simple and easy to operate in a continuous manner.
With this in mind, it should be remembered that a continuous system has potentially, with respect to discontinuous processes, the capacity for providing a greater yield in a given time per unit useful volume of the reactor, a greater possibility of automating the plant and better constancy in the characteristics of the product obtained. The difficulties explained above have, until now, prevented the realisation of advantageous processes for the continuous production of 2,6-dinitro-N-alkyl-anilines.
The object of the present invention is to overcome the disadvantages of the known art relating to the preparation of 2,6-dinitro-N-alkyl-anilines.